Method for the preparation of diamond, graphite or their mixture

ABSTRACT

The present invention provides a method of preparation for diamond, graphite or mixtures of diamond and graphite by reduction of CO or CO 2 . Said method comprises a step of contacting an active metal capable of reducing a carbon source into elementary carbon with carbon source (such as CO and/or CO 2  and/or their origin) under conditions suitable to reduce the carbon source to elementary carbon in the course of a reduction reaction. After the raw diamond or mixtures of diamond and graphite thus obtained are subjected to intensive heat treatment with perchloric acid, pure diamond granules are obtained. The present method employs relatively low reaction temperature and pressure and the facilities needed in the method are simple and easy to operate. Diamond finally obtained has good crystallinity and free of impurities with granule size of several hundred micrometer. In addition, the present invention makes use of the industrial by-product of CO and CO 2  which not only turns wastes into valuables and is low in cost, but also improves the environment and thus possesses both good social benefits and economical benefits.

CROSS-REFERENCE TO RELATED APPLICATION

This Application is a Section 371 National Stage Application ofInternational Application No. PCT/CN2002/000811, filed Nov. 15, 2002 andpublished as WO 2004/046032 A1 on Jun. 3, 2004, and published inChinese, the content of which is hereby incorporated by reference in itsentirety.

FIELD OF INVENTION

The present invention relates to a process for preparing diamond,graphite or mixtures of diamond and graphite from CO or CO₂ served asthe carbon source by reduction with active metals.

BACKGROUND OF THE INVENTION

Diamond has the advantages of high melting point, low compressibilitycoefficient, high symmetry and high refractive index. It has wideapplications in industrial manufacture and scientific research. Owing tothe specific properties and uses thereof, quite long ago, people triedto prepare it by chemical method in order to supplement theinsufficiency of the natural storage. A large amount of time and a longcourse of events have been spent on solving a series of problems such asthe exploration of transition condition and relevant facilities as wellas searches for an effective catalyst. In 1954, first work on successfulpreparation of diamond by conversion of graphite under strict control ofhigh temperature and high pressure with FeS used as the flux wasreported in Nature, Vol. 176, 51. Thereafter research and production ofman-made diamond have been developing rapidly and grows to be a newindustry. The conventional method of preparation for diamond involvesthe use of graphite as the raw material, molten metals (Ni, Cr, Mn, Fe,Co, Ti, Al etc) as the catalyst and flux, little diamond particles ascrystal seeds. Thus graphite is converted into diamond under pressure of5-100 kbar and high temperature of 1200-2400K. This kind of method hasto endure critical conditions and very high cost.

Chinese patent 97119450.5 and Science, 1998, Vol. 281, 246 disclosed amethod in which CCl₄ was used as the carbon source, Na was used as thereducing agent and solvent, Ni—Co metal was used as the catalyst. CCl₄could be converted into diamond at 700° C. The size of the diamondparticles thus prepared was less than 0.2 micrometer and the method hadthe danger of explosion. Therefore at the moment, the method is notsuitable for large-scale industrial production of diamond.

On the other side, the global storage of CO₂ on earth is extremelyabundant. CO₂ is also the by-product of exhaust emission of manyindustrial manufactures. When CO₂ is expelled into air, “greenhouseeffect” will be induced which will cause the global weather gettingwarmer. As a result, many countries in the world have to spend hugeamount of manpower and resources to bring it under control. CO₂ isnon-toxic and cheap. Utilization of CO₂ as main raw material forsynthesizing inorganic and organic compounds is one of the objectives ofchemists. It is regrettable to notice that up to now no anywell-industrialized method of treatment that uses CO₂ as raw material inhuge amount has been reported.

SUMMARY OF THE INVENTION

The objective of the present invention is to provide a method for thepreparation of diamond, graphite or mixtures of diamond and graphite byusing CO₂ (or compound that could release CO₂ on decomposition) or CO(or CO source) as the carbon source and active metals as the reducingagents.

In order to realize the above-mentioned objective, the inventor ofpresent invention carried out a large amount of intensive investigationsand found that CO₂ or CO₂ source and CO or CO source could react withactive metal that is capable of reducing them into elementary carbon toform diamond, graphite or mixture of diamond and graphite.

Hence the present invention provides a method for preparing diamond,graphite or mixture of diamond and graphite. The method includes thesteps in which active metals (capable of reducing carbon source intoelementary carbon) under reducing conditions capable of reducing carbonsource into elementary carbon are brought in contact with carbon source(CO and/or CO source and/or CO₂ and/or CO₂ source) to start a reductionreaction. The carbon source is preferably CO₂ and/or CO₂ source.

In the method of the present invention based on a preferred approach,CO₂ is used as the carbon source and is reduced by active metal to formdiamond. Therefore any compound, such as dry ice, oxalates, carbonatesor their mixture that could release CO₂ on decomposition as well as CO₂itself could be used as a carbon source to prepare diamond.

Any metal that is capable of reducing CO or CO₂ into elementary carboncould be used as an active metal in the present invention. Metals whosestandard electrode potentials are lower than −2.2 V are preferred. Suchmetals include (but not limited to) one or mixture of several of thefollowing: metal Na, Li, K, Rb, Cs, or Mg, Ca, Sr, Ba. Although notwished to be bound by any theory, it is generally believed that standardelectrode potential of CO₂/CO₂ ^(.—) in aprotic solvent is −2.2 V. CO₂^(.—) is an active single electron free radical that reacts easily withCO₂ to form C—C linkage. Therefore those metals having standardelectrode potential lower than −2.2 V, such as the standard electrodepotential of Na is −2.7 V, the standard electrode potential of K is−2.931 V, the standard electrode potential of Li is −3.04 V, thestandard electrode potential of Mg is −2.37 V, could all be used toreduce CO₂ to prepare diamond or mixtures of diamond and graphite.

Temperature of reduction reaction suitable to be used in the presentinvention is preferably 300° C. at least, preferably 300-2000° C.Specific temperature of reaction to be adopted would depend on theselected pressure condition and the selected active metal used. Whenmetal Na or Li, K, Rb, Cs is used as reducing metal, reactiontemperature is preferably 300° C. at least, more preferably 300-2000°C.; When Mg, Ca, Sr, Ba is used as reducing metal, reaction temperatureis preferably 650° C. at least, more preferably 650-2000° C;

Pressure of reduction reaction suitable to be used in the presentinvention is 0.2 kbar at least, preferably 0.2-5.0 kbar. Specificpressure of reaction to be adopted would depend on what kind ofelementary carbon product expected to be prepared and on the temperatureselected. It should be emphasized that when diamond of high purity isexpected to be synthesized, higher pressure is preferably adopted, morepreferably higher pressure is maintained throughout the whole course ofreaction.

Under higher pressure, the product obtained is diamond with highdensity; If the reaction is carried out in a reaction kettle that couldnot maintain higher pressure automatically, pressure of the system willdrop in the course of reaction and the main product formed at that timewill be graphite of lower density and the final product will be amixture of graphite and diamond. Under higher temperature and higherpressure, said reaction route is as follows;3CO₂+4M=C (diamond, graphite or mixture of graphite and diamond)+2M₂CO₃(M=Li or Na, K, Rb, Cs)3CO₂+2N=C (diamond, graphite or mixture of graphite and diamond)+2NCO₃(N=Mg or Ca, Sr, Ba)

The thermodynamic property of diamond determines that a definitepressure is necessary for the formation of diamond. The higher thepressure is, the more favorable the formation of diamond will be. In themethod of the present invention, it is possible to vary the pressure ofthe reaction system by controlling the amounts of dry ice, oxalate,carbonate or CO₂ gas added. Experiments prove that when temperature islower than 300° C. and pressure is lower than 0.2 kbar, no diamond willbe formed. Judging the tolerance of the common reaction kettle, it isappropriate to carry out the reaction at a temperature of 300-2000° C.and a pressure of 0.2-5.0 kbar.

Reduction reaction of the present invention is preferably carried outunder a supercritical condition. It is believed that when CO₂ is heatedto exceed its critical point (for example, 31.5° C., 73 kbar), its gasphase and liquid phase will turn into a single supercritical phasehaving high mixing rate and relatively weaker intermolecular associationpower. This will induce the supercritical CO₂ to possess highreactivity. Many physico-chemical properties of the supercritical CO₂lie between gas and liquid and possess the advantages of both two. Forinstance, it possesses dissolution power and heat conductivitycoefficient similar to liquid and viscosity coefficient and diffusioncoefficient similar to gas. In the preferred approach of the presentinvention, temperature and pressure are adjusted to turn the CO₂ of thereaction system into supercritical state.

Based on the method of the present invention, time of the reductionreaction is determined by temperature, pressure and reducing power ofthe reducing metal adopted. 10-48 hours are preferred.

After the completion of the reaction, the reaction system is cooled toroom temperature, and pressure is lowered to atmospheric pressure anddiamond, graphite or their mixture could be obtained.

If small diamond granule, such as 300 μm-sized diamond granule, is addedto the above-mentioned reaction system as a crystal seed, up to 6000μm-sized diamond granule could be obtained. For the sake of convenienceand lowering cost, reaction product of the preceding experiment ispreferably selected as the crystal seed.

In order to obtain pure diamond, diamond or mixture of diamond andgraphite obtained by the method of the present invention could bepurified by any conventional purification method. For instance, it ispossible to obtain pure diamond particulate through intensive heattreatment with perchloric acid or sedimentation separation with 0.5%aqueous solution of gum Arabic.

If only graphite is to be prepared, reaction could be carried out merelyat pressure lower than 0.2 kbar.

The present invention utilizes industrial by-product CO₂ or CO orcompounds capable of releasing CO or CO₂ on decomposition as the mainraw material and thus possesses the advantages of low reactiontemperature, good dispersion and good flowability of carbon source.Diamond crystals obtained have good crystallinity, contains noimpurities and could have size up to several hundred micrometers. Ifsmall diamond granule, such as 300 μm-sized diamond granule, is added tothe above-mentioned reaction system as a crystal seed, diamond granulewith size of 3000 μm or even up to 6000 μm could be obtained.

Especially when CO₂ is used as the carbon source, the approach has thefollowing advantages: CO₂ is the by-product of exhaust emission of manyindustrial manufactures. When CO₂ is expelled into air, “greenhouseeffect” will be induced which will cause the global weather gettingwarmer. As a result, many countries in the world have to spend hugeamount of manpower and resources to bring it under control. The presentinvention uses CO₂ as the raw material to prepare diamond, graphite ortheir mixture. Thus this approach not only could turn wastes intovaluables and could perform it at low cost but also it is beneficial tothe improvement of environment and thus possesses good social benefitand economic benefit. The present invention could also be used to reduceCO₂ into graphite that is also an important industrial raw material.Using CO₂ as a reactant has advantages of non-toxicity, non-combustionand safe handling. In addition, CO₂ could easily be separated from thereactant and thus by lowering the pressure, CO₂ turns into gas and thenis expelled and the final product could conveniently and directly beobtained.

In comparison with the conventional method of preparation for diamond,the present method uses lower temperature, pressure, simpler facilityand is of low cost and easily manipulative. When compared with method ofreduction of CCl₄ into diamond, the present method is safer inmanipulation, could yield diamond of larger size and thus possessessignificance of practical industrial production and potential widemarkets.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the X-ray diffraction profile for the sample obtained inExample 1.

FIG. 2 is the Raman spectrum for the sample obtained in Example 1.

FIG. 3 a, 3 b are SEM (scanning electronic micrograph) profiles for thediamond sample obtained in Example 1. FIG. 3 b is a magnified profilefor the portion selected by the square in FIG. 3 a.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

2.0 g of metal sodium of chemical pure grade and 8.0 g of self-prepareddry ice were put into an autoclave of a capacity of 12 mL. The autoclavewas heated to 440° C. so that the pressure in the autoclave reached500-1000 kbar and this state was maintained for 16 hrs. Then theautoclave was cooled to room temperature and the pressure in theautoclave dropped to atmospheric pressure. The reaction product wastreated with HCl and washed with water. 0.20 g of black powder wasobtained.

The X-ray diffraction spectrum of the sample was measured. In thediffraction spectrum (FIG. 1) of the sample obtained, there appeared 3characteristic diffraction peaks of cubic phase diamond (JCPDS card No.6-675) and 1 rather broad diffraction peak of graphite at 26.2°.

Raman spectrum of the sample was measured. In the spectrum, there was acharacteristic peak of diamond at 1332 cm⁻¹ (FIG. 2, 1332 cm⁻¹ is thecharacteristic peak of diamond, see Nature 1999, Vol. 402, 164) with itshalf-height width of 4.7 cm⁻¹ close to that of natural diamond (2.5cm⁻¹), which indicating that the diamond synthesized has goodcrystallinity. In addition there were two characteristic peaks ofgraphite at 1363 cm⁻¹ and 1591 cm⁻¹ respectively indicating that theproduct was a mixture of diamond and graphite.

The mixed powder obtained was intensively heat-treated with perchloricacid at 160° C. and 0.018 g of pure diamond granule was finallyobtained. SEM micrograph indicated that the average diameter of thediamond granule was 100 μm (FIG. 3).

If the metal sodium of the present example was replaced by Li, K, Rb, Csas the reducing metal, mixtures of diamond and graphite were similarlyobtained.

EXAMPLE 2

2.5 g of potassium of chemical pure grade was put into an autoclave.Said autoclave was heated to 470° C. and CO₂ gas was fed into anautoclave under pressure to 400-1500 kbar and this state was maintainedfor 12 hrs. Then the autoclave was cooled to room temperature and thepressure in the autoclave dropped to atmospheric pressure. The reactionproduct was treated with HCl and washed with water. 0.22 g of blackpowder was obtained.

X-ray diffraction pattern and Raman spectrum of the sample were measuredand the obtained sample was proved to be a mixture of diamond andgraphite.

The mixed powder obtained was separated by sedimentation in 0.5% aqueoussolution of gum Arabic and 0.02 g of pure diamond granule was finallyobtained. SEM micrograph indicated that the average diameter of thediamond granule was 120 μm and the maximum diameter could reach 300 μm.

If potassium of the present example was replaced by Li, Na, Rb, Cs asthe reducing metal, mixture of diamond and graphite was similarlyobtained.

EXAMPLE 3

2.2 g of potassium of chemical pure grade and 6.0 g of MgCO₃ were putinto an autoclave of 12 mL that was heated to 500° C. and to a pressureof 800-2000 kbar and this state was maintained for 18 hrs. Then theautoclave was cooled to room temperature and the pressure in theautoclave dropped to atmospheric pressure. 0.08 g of black powder wasobtained.

X-ray diffraction pattern and Raman spectrum of the sample were measuredand the obtained sample was proved to be a mixture of diamond andgraphite.

The mixed powder obtained was intensively heat treated with perchloricacid and pure diamond granule with average diameter of the diamondgranule of 260 μm (determined by SEM) was obtained.

If potassium of the present example was replaced by Li, Na, Rb, Cs asthe reducing metal, mixtures of diamond and graphite were similarlyobtained.

If MgCO₃ of the present example was replaced by Ag₂CO₃, CaCO₃, CdCO₃,CoCO₃, CuCO₃,FeCO₃, BaCO₃, MnCO₃, NiCO₃, PbCO₃, SrCO₃, ZnCO₃, Na₂CO₃,K₂CO₃, Li₂CO₃ and the temperature was changed to 470° C., 950° C., 500°C., 450° C., 480° C., 520° C., 1000° C., 460° C., 550° C., 540° C., 900°C., 440° C., 1500° C., 1400° C., 750° C. respectively, a mixture ofdiamond and graphite was similarly obtained.

EXAMPLE 4

2.2 g of Li of chemical pure grade and 14.0 g of NiC₂O₄ were put into anautoclave of 12 mL which was heated to 560° C. and to a pressure of500-1000 kbar and this state was maintained for 12 hrs. Then theautoclave was cooled to room temperature and the pressure in theautoclave dropped to atmospheric pressure. The reaction product wastreated with HCl and washed with water. 0.28 g of black powder wasobtained.

X-ray diffraction pattern and Raman spectrum of the sample were measuredand the obtained sample was proved to be a mixture of diamond andgraphite.

The mixed powder obtained was intensively heat treated with perchloricacid at 160° C. and pure diamond granule with average diameter of thediamond granule of 100 μm (determined by SEM) was obtained.

If Li of the present example was replaced by K, Na, Rb, Cs as thereducing metal, a mixture of diamond and graphite was similarlyobtained.

If NiC₂O₄ of the present example was replaced by CaC₂O₄, CdC₂O₄, CoC₂O₄,CuC₂O₄, CrC₂O₄, FeC₂O₄, K₂C₂O₄, MnC₂O₄, La₂(C₂O₄)₃, Li₂C₂O₄, MgC₂O₄,Na₂C₂O₄, PbC₂O₄, SrC₂O₄, ZnC₂O₄, La₂(C₂O₄)₃, Cr₂(C₂O₄)₃, a mixture ofdiamond and graphite was similarly obtained.

EXAMPLE 5

2.5 g of Mg of chemical pure grade was put into an autoclave of 12 mL.The autoclave was heated to 650° C. and CO₂ gas was fed into anautoclave under pressure to 500-1500 kbar and this state was maintainedfor 12 hrs. Then the autoclave was cooled to room temperature and thepressure in the autoclave dropped to atmospheric pressure. The reactionproduct was treated with HCl and washed with water. 0.23 g of blackpowder was obtained.

X-ray diffraction pattern and Raman spectrum of the sample were measuredand the obtained sample was proved to be a mixture of diamond andgraphite.

The mixed powder obtained was intensively heat treated with perchloricacid and a pure diamond granule was finally obtained. SEM micrographindicated that the average diameter of the diamond granule was 60 μm.

If Mg of the present example was replaced by Ca, Sr, Ba as the reducingmetal and temperature was changed to 850° C., 800° C. and 750° C.respectively, mixture of diamond and graphite was similarly obtained.

EXAMPLE 6

2.5 g of Ca of chemical pure grade and 8.0 g of self-prepared dry icewere put into an autoclave of capacity of 12 mL. The autoclave washeated to 850° C. so that the pressure in the autoclave reached 500-1000kbar and this state was maintained for 16 hrs. Then the autoclave wascooled to room temperature and the pressure in the autoclave dropped toatmospheric pressure. The reaction product was treated with HCl andwashed with water. 0.20 g of black powder was obtained.

X-ray diffraction pattern and Raman spectrum of the sample were measuredand the obtained sample was proved to be a mixture of diamond andgraphite.

The mixed powder obtained was intensively heat treated with perchloricacid and pure diamond granule was finally obtained. SEM micrographindicated that the average diameter of the diamond granule was 130 μm.

If Mg of the present example was replaced by Ca, Sr, Ba as the reducingmetal and temperature was changed to 850° C., 800° C. and 750° C.respectively, mixture of diamond and graphite was similarly obtained.

EXAMPLE 7

2.0 g of Mg of chemical pure grade and 14.0 g of CoC₂O₄ were put into anautoclave of 12 mL which was heated to 650° C. and to a pressure of500-1000 kbar and this state was maintained for 16 hrs. Then theautoclave was cooled to room temperature and the pressure in theautoclave dropped to atmospheric pressure. The reaction product wastreated with HCl and washed with water. 0.20 g of black powder wasobtained.

X-ray diffraction pattern and Raman spectrum of the sample were measuredand the obtained sample was proved to be a mixture of diamond andgraphite.

The mixed powder obtained was intensively heat treated with perchloricacid and pure diamond granule with average diameter of the diamondgranule of 50 μm (determined by SEM) was obtained.

If Mg of the present example was replaced by Ca, Sr, Ba as the reducingmetal and the temperature was changed to 850° C., 750° C., 800° C.,mixture of diamond and graphite was similarly obtained.

If CoC₂O₄ of the present example was replaced by CaC₂O₄, CdC₂O₄, NiC₂O₄,CuC₂O₄, CrC₂O₄, FeC₂O₄, K₂C₂O₄, MnC₂O₄, La₂(C₂O₄)₃, Li₂C₂O₄, MgC₂O₄,Na₂C₂O₄, PbC₂O₄, SrC₂O₄, ZnC₂O₄, mixture of diamond and graphite wassimilarly obtained.

EXAMPLE 8

3.5 g of Sr of chemical pure grade and 16.0 g of FeCO₃ were put into anautoclave of 12 mL which was heated to 800° C. and to a pressure of500-1500 kbar and this state was maintained for 16 hrs. Then theautoclave was cooled to room temperature and the pressure in theautoclave dropped to atmospheric pressure. The reaction product wastreated with HCl and washed with water. 0.28 g of black powder wasobtained.

X-ray diffraction pattern and Raman spectrum of the sample were measuredand the obtained sample was proved to be a mixture of diamond andgraphite.

The mixed powder obtained was intensively heat treated with perchloricacid and pure diamond granule with average diameter of the diamondgranule of 100 μm (determined by SEM) was obtained.

If Sr of the present example was replaced by Ca, Mg, Ba as the reducingmetal and the temperature was changed to 850° C., 650° C., 800° C.,mixture of diamond and graphite was similarly obtained.

If FeCO₃ of the present example was replaced by CaCO₃, CdCO₃, CoCO₃,CuCO₃, Mg CO₃, BaCO₃, MnCO₃, NiCO₃, PbCO₃, SrCO₃, ZnCO₃, Na₂CO₃, K₂CO₃,Li₂CO₃ and the temperature was changed to 950° C., 820° C., 840° C.,880° C., 860° C., 1000° C., 860° C., 850° C., 840° C., 900° C., 940° C.,1500° C., 1400° C., 850° C., mixture of diamond and graphite wassimilarly obtained.

EXAMPLE 9

2.2 g of K of chemical pure grade was put into an autoclave and adiamond seed of size of 300 μm was added. The autoclave was heated to520° C. and CO₂ gas was fed into autoclave under pressure to 500-1500kbar and this state was maintained for 16 hrs. Then the autoclave wascooled to room temperature and the pressure in the autoclave dropped toatmospheric pressure. The reaction product was treated with HCl andwashed with water. 0.24 g of black powder was obtained.

X-ray diffraction pattern and Raman spectrum of the sample were measuredand the obtained sample was proved to be a mixture of diamond andgraphite.

The mixed powder obtained was intensively heat treated with perchloricacid and a pure diamond granule was finally obtained. SEM micrographindicated that the average diameter of the diamond granule was 430 μm.

If K of the present example was replaced by Li, Na, Rb, Cs as thereducing metal, mixture of diamond and graphite was similarly obtained.

EXAMPLE 10

3.2 g of Cs of chemical pure grade and 8.0 g of self-prepared dry icewere put into an autoclave of a capacity of 12 mL and a diamond seed of300 μm was also added. After the autoclave was heated to 300° C., CO₂gas was fed under pressure, so that the pressure in the autoclavereached 200-1500 kbar and this state was maintained for 16 hrs. Then theautoclave was cooled to room temperature and the pressure in theautoclave dropped to atmospheric pressure. The reaction product wastreated with HCl and washed with water. 0.12 g of black powder wasobtained.

X-ray diffraction pattern and Raman spectrum of the sample were measuredand the obtained sample was proved to be a mixture of diamond andgraphite.

The mixed powder obtained was intensively heat treated with perchloricacid and a pure diamond granule was finally obtained. SEM micrographindicated that the average diameter of the diamond granule was 300 μm.

If Cs of the present example was replaced by Li, Na, Rb, K as thereducing metal and temperature was changed to 450° C., 520° C., 480° C.and 580° C. respectively, mixture of diamond and graphite was similarlyobtained.

EXAMPLE 11

2.2 g of potassium of chemical pure grade, 6.0 g of MgCO₃ and diamondseed of 300 μm were put into an autoclave of 12 mL which was heated to500° C. and to a pressure of 800-1000 kbar and this state was maintainedfor 18 hrs. Then the autoclave was cooled to room temperature and thepressure in the autoclave dropped to atmospheric pressure. The reactionproduct was treated with HCl and washed with water. 0.10 g of blackpowder was obtained.

X-ray diffraction pattern and Raman spectrum of the sample were measuredand the obtained sample was proved to be a mixture of diamond andgraphite.

The mixed powder obtained was intensively heat treated with perchloricacid and a diamond granule with average diameter of 270 μm (determinedby SEM) was obtained.

If potassium of the present example was replaced by Li, Na, Rb, Cs asthe reducing metal, mixture of diamond and graphite was similarlyobtained.

If MgCO₃ of the present example was replaced by CaCO₃, CdCO₃, CoCO₃,CuCO₃, FeCO₃, BaCO₃, MnCO₃, NiCO₃, PbCO₃, SrCO₃, ZnCO₃, Na₂CO₃, K₂CO₃,Li₂CO₃ and the temperature was changed to 950° C., 500° C., 450° C.,480° C., 520° C., 1000° C., 460° C., 550° C., 540° C., 900° C., 440° C.,1500° C., 1400° C., 750° C. respectively, mixture of diamond andgraphite was similarly obtained.

EXAMPLE 12

2.2 g of Na of chemical pure grade, 16.0 g of NiC₂O₄ and diamond seed of300 μm were put into an autoclave of 12 mL which was heated to 480° C.and to a pressure of 500-1000 kbar and this state was maintained for 18hrs. Then the autoclave was cooled to room temperature and the pressurein the autoclave dropped to atmospheric pressure. The reaction productwas treated with HCl and washed with water. 0.26 g of black powder wasobtained.

X-ray diffraction pattern and Raman spectrum of the sample were measuredand the obtained sample was proved to be a mixture of diamond andgraphite.

The mixed powder obtained was intensively heat treated with perchloricacid and pure diamond granule with average diameter of the diamondgranule of 360 μm was obtained.

If Na of the present example was replaced by Li, Na, Rb, Cs as thereducing metal, mixtures of diamond and graphite were similarlyobtained.

If NiC₂O₄ of the present example was replaced by CaC₂O₄, CdC₂O₄, CoC₂O₄,CuC₂O₄, CrC₂O₄, FeC₂O₄, K₂C₂O₄, MnC₂O₄, La₂(C₂O₄)₃, Li₂C₂O₄, MgC₂O₄,Na₂C₂O₄, PbC₂O₄, SrC₂O₄, ZnC₂O₄, mixture of diamond and graphite wassimilarly obtained. Oxalates that could release CO₂ on decompositioncould also be used as the carbon source for producing a diamond.

EXAMPLE 13

2.5 g of Mg of chemical pure grade and reaction product of Example 2(used as a seed) were put into an autoclave. The autoclave was heated to650° C. and CO₂ gas was fed into autoclave under pressure to 500-1500kbar and this state was maintained for 12 hrs. Then the autoclave wascooled to room temperature and the pressure in the autoclave dropped toatmospheric pressure. The reaction product was treated with HCl andwashed with water. 0.24 g of black powder was obtained.

X-ray diffraction pattern and Raman spectrum of the sample were measuredand the obtained sample was proved to be a mixture of diamond andgraphite.

The mixed powder obtained was intensively heat treated with perchloricacid and pure diamond granule was finally obtained. SEM micrographindicated that the average diameter of the diamond granule was 3200 μm.

If Mg in the present example was replaced by Ca, Sr, Ba as the reducingmetal and the temperature was changed to 860° C., 840° C., 780° C.,mixture of diamond and graphite was similarly obtained.

EXAMPLE 14

2.0 g of Sr of chemical pure grade, 8.0 g of self-prepared dry ice anddiamond seed of 300 μm were put into an autoclave of a capacity of 12mL. The autoclave was heated to 800° C. so that the pressure in theautoclave reached 500-1000 kbar and this state was maintained for 16hrs. Then the autoclave was cooled to room temperature and the pressurein the autoclave dropped to atmospheric pressure. The reaction productwas treated with HCl and washed with water. 0.21 g of black powder wasobtained.

X-ray diffraction pattern and Raman spectrum of the sample were measuredand the obtained sample was proved to be a mixture of diamond andgraphite.

The mixed powder obtained was intensively heat treated with perchloricacid and pure diamond granule was finally obtained. SEM micrographindicated that the average diameter of the diamond granule was 1100 μm.

If Sr of the present example was replaced by Ca, Mg, Ba as the reducingmetal and temperature was changed to 880° C., 680° C. and 820° C.respectively, mixtures of diamond and graphite were similarly obtained.

EXAMPLE 15

2.0 g of Mg of chemical pure grade, 14.0 g of FeC₂O₄ and diamond seed of300 μm were put into an autoclave of 12 mL which was heated to 700° C.and to a pressure of 500-1000 kbar and this state was maintained for 16hrs. Then the autoclave was cooled to room temperature and the pressurein the autoclave dropped to atmospheric pressure. The reaction productwas treated with HCl and washed with water. 0.20 g of black powder wasobtained.

X-ray diffraction pattern and Raman spectrum of the sample were measuredand the obtained sample was proved to be a mixture of diamond andgraphite.

The mixed powder obtained was intensively heat treated with perchloricacid and pure diamond granule with average diameter of 800 μm wasobtained.

If Mg of the present example was replaced by Ca, Sr, Ba as the reducingmetal and the temperature was changed to 860° C., 840° C., 780° C.,mixture of diamond and graphite was similarly obtained.

If FeC₂O₄ of the present example was replaced by CaC₂O₄, CdC₂O₄, CoC₂O₄,CuC₂O₄, CrC₂O₄, NiC₂O₄, K₂C₂O₄, MnC₂O₄, La₂(C₂O₄)₃, Li₂C₂O₄, MgC₂O₄,Na₂C₂O₄, PbC₂O₄, SrC₂O₄, ZnC₂O₄, mixture of diamond and graphite wassimilarly obtained.

EXAMPLE 16

2.0 g of Ca of chemical pure grade, 16.0 g of FeCO₃ and diamond seed of300 μm were put into an autoclave of 12 mL which was heated to 850° C.and to a pressure of 500-1000 kbar and this state was maintained for 16hrs. Then the autoclave was cooled to room temperature and the pressurein the autoclave dropped to atmospheric pressure. The reaction productwas treated with HCl and washed with water. 0.20 g of black powder wasobtained.

X-ray diffraction pattern and Raman spectrum of the sample were measuredand the obtained sample was proved to be a mixture of diamond andgraphite.

The mixed powder obtained was intensively heat treated with perchloricacid and pure diamond granule with an average diameter of 1600 μm(determined by SEM) was obtained.

If Ca of the present example was replaced by Mg, Sr, Ba as the reducingmetal and the temperature was changed to 660° C., 880° C., 820° C.,mixtures of diamond and graphite were similarly obtained.

If FeCO₃ of the present example was replaced by CaCO₃, CdCO₃, CoCO₃,CuCO₃, MgCO₃, BaCO₃, MnCO₃, NiCO₃, PbCO₃, SrCO₃, ZnCO₃, Na₂CO₃, K₂CO₃,Li₂CO₃ and the temperature was changed to 950° C., 860° C., 870° C.,880° C., 920° C., 1000° C., 860° C., 950° C., 850° C., 900-C, 880° C.,1500° C., 1400° C., 850° C., respectively, mixture of diamond andgraphite was similarly obtained.

1. A method of preparation for diamond, graphite or mixtures of diamondand graphite, comprising a step of contacting an active metal capable ofreducing carbon source to elementary carbon with a carbon source of COand/or CO₂ and/or its origin under conditions enough to reduce carbonsource into elementary carbon so as to carry out a reduction reaction.2. The method as claimed in claim 1, wherein said carbon source is CO₂or CO₂ source or their mixtures.
 3. The method as claimed in claim 2,wherein said CO₂ includes dry ice and said CO₂ source includes oxalates,carbonates or their mixtures.
 4. The method as claimed in claim 2,wherein said active metal is a metal having standard electrode potentiallower than −2.2 V.
 5. The method as claimed in claim 4, wherein saidactive metals are at least one selected from a group consisting of Na,Li, K, Rb, Cs, Mg, Ca, Sr and Ba.
 6. The method as claimed in claim 5,wherein when said active metals are at least one selected from a groupconsisting of Na, Li, K, Rb and Cs, the temperature of the reductionreaction should at be at least 300° C.; when said active metal is atleast one selected from a group consisting of Mg, Ca, Sr and Ba, thetemperature of the reduction reaction should be at least 650° C.
 7. Themethod as claimed in claim 1, wherein the pressure of the reductionreaction should be at least 0.2 kbar.
 8. The method as claimed in claim1, further comprising a step of adding diamond particulate as crystalseed into the reaction system before the start of the reaction.
 9. Themethod as claimed in claim 1, further comprising a step of subjectingraw diamond or mixture of diamond and graphite to a purification processto yield pure diamond granules.
 10. The method as claimed in claim 9,wherein said purification processes could be carried out either byintensive heat treatment with perchloric acid or by sedimentationseparation in aqueous solution of gum Arabic.